July 1st, 18S7.] 



SCIENTIFIC NEWS. 



109 



THE SPECTRUM.— II. 



IF the light which Newton used in the important experi- 

 ment, described on page 86, had consisted of a few 

 separate colours, he would have seen as many round spots 

 of differently-coloured light upon the screen. But there 

 were innumerable colours, and they overlapped each other, 

 forming a continuous band. Had one or two tints been 

 missing, their absence would not have been noticed on 

 account of this overlapping. 



In 1802 the experiment was repeated by Dr. Wollaston, 

 the eccentric physician who used a tailor's thimble for a 

 galvanic battery, and a few watch-glasses in a tea-tray for a 

 chemical laboratory, and who never permitted even his most 



Marshall, in 1847, not only noticed the yellow grains in the 

 mill-stream, but recognised their worth. 



Wollaston saw the black lines, and was puzzled by them, 

 but a German optician twelve years later examined the 

 spectrum with great care, and mapped down 576 lines, which 

 are called after him — Fraunhofer lines. He indicated the 

 most prominent by the letters A, B, C, D, E, F, G, and H. 

 The importance of his contribution to the work lies in his 

 having pointed out that these dark lines are invariably pre- 

 sent in the spectrum of sunlight, and in having shown, by 

 accurate measurements, that they held invariable positions, 

 the distance between any of them being always the same. 

 He observed the spectrum of the moon and of Venus, and 

 found the same lines present, the light being merely reflected 



Fig. 4- 



intimate friends to enter his study. Using a narrow hole or 

 slit, he discovered that the coloured band, or spectrum, 

 which had for 130 years been supposed to be continuous, 

 was crossed by a multitude of fine lines. Some of the 

 colours were, in fact, wanting, like missing skeins in a 

 collection of threads of every hue. 



It is somewhat surprising that Newton did not make this 

 discovery, for he carried out a large number of experiments 

 with prisms, and recognised the importance of a pure spec- 

 trum, and found that this could not be produced unless the 

 prism was arranged at such an angle that the light passed 

 through with a minimum deviation from its original path ; 



Fig. 5- 



he found, too, that when this position was attained, the 

 angle at which the ray struck the first face of the glass was 

 equal to that at which it left the second face, and that, in fact, 

 the minimum deviation can be arranged for only one part of 

 the spectrum at a time. 



We know that he tried holes ot different sizes ; he must 

 have found that a large hole produced an impure spectrum, 

 and that a smaller one gave a better, though fainter, result. 

 He was probably nearer a great discovery, and missed it by 

 less than did Captain Soutter, who grazed his immense flocks 

 and herds on the Californian goldfields, and whose friend 



sunlight. He found that ordinary flame gives a continuous 

 spectrum free from dark lines. He therefore came to the 

 conclusion that these dark lines, in whatever way they were 

 produced, were due to some property of the sun, and had 

 nothing to do with any imperfection in his instruments. 

 He does not appear to have had any idea of their cause; he 

 did not utilise them, as has since been done, for carrying out 

 any discovery, but they will ever bear the name of Fraun- 

 hofer lines as a tribute to his accurate and patient work. 



Fig. 4 represents some of the more prominent lines of 

 the sun's spectrum. The scale shows the wave length. It 

 will be noticed that the violet end is considerably extended 

 compared with the red end, the spectrum represented being 

 one produced by a prism. A diffraction spectrum from a 

 grating would have had a uniform scale, the D line being 

 almost exactly in the middle. The wave lengths are 

 generally measured in millionths of a millimetre, for 

 example the B line has a wave length of 686-7 millionths. 

 Those who are not familiar with millimetres may form an 

 idea of its size by noticing the thickness of a half-sovereign, 

 and those who are not familiar with half-sovereigns may 

 refer to the black line surrounding the advertisements on 

 the cover of this journal, this line being about one milli- 

 metre wide. 



With a powerful spectroscope the line A is found to be 

 a broad band in deep red, forming the nucleus of a number 

 of lines. B is a single line with two fainter ones very close 

 to it. Between A and B is a cluster of lines indicated by 

 one line only in Fig. 4. The spectrum is practically con- 

 tinuous up to C, a single distinct line. After a series of 

 faint lines we come to the most remarkable of all, the D 

 line ; this is double, but needs a fine slit and a fair magni- 

 fying power to show it thus. The corresponding wave 

 lengths are 589-5 and 5S8-9 millionths of a millimetre. It 

 is in the yellow part of the spectrum. E is a group of fine 

 lines in the green. A rather remarkable group a little 

 further on has been named b. F is a strongly marked line in 

 the blue. A number of small lines cross the spectrum after 

 this, and cluster thickly not only in clear black lines, but in 

 cloudy bands round G. The rest of the spectrum is very full 



